tree-optimization/98640 - fix bogus sign-extension with VN

tree-optimization/98640 - fix bogus sign-extension with VN

VN tried to express a sign extension from int to long of
a trucated quantity with a plain conversion but that loses the
truncation.  Since there's no single operand doing truncate plus
sign extend (there was a proposed SEXT_EXPR to do that at some
point mapping to RTL sign_extract) don't bother to appropriately
model this with two ops (which the VN insert machinery doesn't
handle and which is unlikely to CSE fully).

2021-01-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98640
	* tree-ssa-sccvn.c (visit_nary_op): Do not try to
	handle plus or minus from a truncated operand to be
	sign-extended.

	* gcc.dg/torture/pr98640.c: New testcase.